1. Field of the Invention
The invention relates to integrated circuits, and in particular to a light emitting diode driver circuit that includes on-board bias and dimming control settings.
2. Related Art
A light emitting diode (LED) is a diode that emits photons in response to a current flow between its anode and cathode. LEDs are often used in modern lighting applications due to their durability, efficiency, and small size compared to other light sources.
The two main characteristics of LED output are spectral distribution and optical intensity. “Spectral distribution” refers to the distribution of light wavelengths in a particular frequency band of the LED output while “optical intensity” refers to the overall brightness of the LED output. The values of these output characteristics-are controlled by a set of LED drive parameters. For example, the LED drive parameter that controls the spectral distribution of a LED output is bias current (i.e., the current flowing through the LED). Optical intensity can also be controlled by bias current, but since changing the bias current changes the spectral distribution of the LED output, using bias current as a drive parameter for brightness control is often unacceptable.
Therefore, to adjust the optical intensity of a LED while maintaining the desired spectral distribution, pulse width modulation (PWM) is usually employed. PWM involves regulating the bias current through the LED so that the current switches between zero and the optimal bias current. By increasing or decreasing the duty cycle (i.e., the percentage of time a bias current is actually flowing through the LED in a given period) of this switching, the optical intensity of the LED output can be increased or decreased, respectively, without changing the spectral density of the LED output. By cycling at a high enough frequency, visible flickering of the LED output can be avoided.
To properly drive LEDs in modern LED applications, LED driver ICs (integrated circuits) are commonly used. A LED driver IC includes circuitry that allows for accurate control over a desired set of LED drive parameters (e.g., bias current and duty cycle) for a LED or group of LEDs. Note that because LEDs are current controlled devices, voltage is not considered a LED drive parameter. The voltage drop across any given LED or group of LEDs is determined by the LEDs themselves, and cannot actually be controlled by the LED driver IC.
FIG. 1 shows a conventional LED circuit 100 formed on a board 101. LED circuit 100 includes a LED driver IC 103, such as the LINEAR TECHNOLOGY™ LT1932 LED driver IC, which includes an input voltage pin VIN, a switching pin SW, a LED drive pin DRV, a shutdown pin SHDN, a current set pin RSET, and a ground pin GND. LED driver IC 103 drives a string of LEDs LS1 via LED drive pin DRV.
To generate the voltage required by LED string LS1, LED driver IC 103 includes switching circuitry that periodically shorts an inductor L1 to ground via switching pin SW. This allows energy (from supply voltage VIN) to be stored in the magnetic field of inductor L1. When the short is removed, the combined voltage from inductor L1 and input voltage VSOURCE charges a capacitor C2 to provide an elevated voltage VBOOST at node A, thereby providing an elevated voltage that satisfies the forward voltage requirements of LED string LS1.
The specific values for the LED drive parameters that are applied to LED string LS1 by LED driver IC 103 are determined by a set of external (i.e., off chip) components, including a resistor R1 and a dimming circuit 102, which are both mounted on a printed circuit board (PCB) 101. For example, the bias current that flows through LED string LS1 is determined by a programming current that flows out of set pin RSET. Resistor R1, which is connected between current set pin RSET and ground, determines the magnitude of this programming current. The higher the resistance of resistor R1, the lower the programming current, and the lower the current flow through LED string LS1.
The optical intensity of the output from LED string LS1 can be adjusted via shutdown pin SHDN. A PWM signal PWM_CTRL from dimming logic 102 applied directly to shutdown pin SHDN  causes LED driver IC 103 to apply the same on/off duty cycle to LED drive pin DRV, thereby pulsing LED string LS1 at the same rate as PWM signal PWM_CTRL. By increasing or decreasing the duty cycle of PWM signal PWM_CTRL the brightness of the output from LED string LS1 can be increased or decreased, respectively.
In this manner, the components of LED circuit 100 that are external to LED driver IC 103 ensure that LED driver IC 103 applies a desired set of LED drive parameter values to LED string LS1. As a result, LED string LS1 is caused to produce a LED output having a desired spectral density and optical intensity.
Note that while different LED driver ICs may use different sets of external components, all conventional LED driver ICs require some type of external circuitry for setting LED drive parameter values. Unfortunately, those external components can complicate the assembly and limit the minimum size of LED circuits that include conventional LED driver ICs.
In an effort to remove some of the size constraints associated with LED driver ICs, the ADVANCED ANALOGIC TECHNOLOGIES™ AAT3113 and AAT3114 LED driver ICs include a bias current module that can be programmed by an external programming signal. However, because the AAT3113/4 LED driver ICs require the external programming signal each time the chip is powered up, the responsiveness of those LED driver ICs is compromised. For example, “instant on” operation is not possible since the AAT3113/4 LED driver ICs must wait for the programming signal before it can provide the desired bias current. Furthermore, the need for a signal source to provide the programming signal (or a control signal such as a PWM signal) can significantly complicate the overall LED circuit design.
Accordingly, it is desirable to provide a LED driver IC that minimizes area requirements and can operate without external control signals or external components.